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"Drive may be designed for constant torque or for torque varying as the square of the speed.

"Drive may be designed for constant torque or for torque varying as the square of the speed.

FIGURE 8 Block diagram for wound-rotor induction motor with magnetic secondary control power (General Electric)

Figure 8 is a three-phase resistor connected across the slip rings of the motor. Its resistance rating is selected to provide minimum motor torque at standstill or adequate torque at minimum speed. Contacts in the form of magnetic contactors, drum, cam, or dial switches are closed by a logic device (generally magnetic) to short-circuit part or all of the resistor. Thus, instead of a modulated or infinitely variable resistor as encountered with the other two drives, this drive modifies secondary resistance in discrete steps, providing discrete speed-torque curves instead of an infinite family. The number of speed-torque curves the control generates will depend directly on the number of contacts provided in the secondary-control circuit.

Either a manual switch or some automatic contact-making device actuated by the process can be utilized to actuate the control logic.

Secondary controls are normally packaged in the factory with resistors and contact-making secondary controller integrally assembled.

TABLE 4 Wound-rotor inductions motor with contact-type secondary control drive data
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